1. Field of the Invention
The present invention relates to a separator for fuel cell for use in portable power supply, power supply for electric car, household cogeneration system, etc., a fuel cell, a method of producing a fuel cell separator, an apparatus of producing a fuel cell separator and a mold for use in the production of the fuel cell separator.
2. Related Art of the Invention
A fuel cell comprising a solid polymer electrolyte undergoes electrochemical reaction of a fuel gas containing hydrogen with a fuel gas containing oxygen such as air to generate electric power and heat at the same time. The configuration of the solid polymer electrolyte will be described below.
Firstly, a catalytic reaction layer mainly composed of a carbon powder having a platinum group metal catalyst supported thereon is formed on the both sides of a polymer electrolyte membrane which selectively transports hydrogen ion.
Subsequently, a diffusion layer having a permeability to fuel gas and an electronic conductivity in combination is formed on the outer surface of the catalytic reaction layer. The diffusion layer and the catalytic reaction layer together form an electrode. The combination of the polymer electrolyte membrane and the electrode is called as MEA (membrane-electrode assembly).
Subsequently, a gas sealing material or gasket is provided around the electrodes with the polymer electrolyte membrane interposed therebetween such that the fuel gas supplied cannot leak from MEA to the exterior or the two fuel gases cannot be mixed with each other. The sealing material or gasket has been previously integrated to the electrode and the polymer electrolyte membrane. The assembly thus integrated is called as MESA (membrane-electrode-seal assembly).
MESA has a separator disposed on the outer side thereof. This separator acts to mechanically fix MESA and connect adjacent MEA's to each other in series. A gas flow path is formed at the site where the separator comes in contact with MEA. This gas flow path is adapted to supply a reactive gas onto the surface of the electrode and carry out produced gas, extra gas or water produced by reaction away. The gas flow path may be provided separately of the separator but is normally formed by providing a groove on the surface of the separator.
In order to supply a fuel gas into the groove provided on the surface of the separator, pipe fittings are needed. In other words, these pipe fittings are adapted to branch the fuel gas piping into pipings by the number corresponding to the number of the separators used and connect these branches directly to the grooves on the separator. This pipe fitting is called manifold. The type of the manifold which connects the aforementioned fuel gas supplying piping directly to the groove on the separator is called external manifold. There is also a type of manifold having a simpler configuration called internal manifold. The internal manifold is formed by providing a through-hole in the separator having a gas flow path formed thereon. The inlet and outlet of the gas flow path are disposed in the hole so that the fuel gas can be directly supplied from the hole into the gas flow path.
Since a fuel cell generates heat during operation, cooling medium such as water is necessary for the fuel cell to be kept in a suitable temperature state. In general, a cooling portion is provided interposed between separators every 1 to 3 units of fuel cell. In this case, the cooling portion is mostly provided by forming a cooling medium flow path on the back surface of the separator. These MEA's, separators and cooling portions are alternately stuck with 10 to 200 cells. Thereafter, the laminate of MEA, separator and cooling portion is clamped between end plates with a collector and an insulating plate interposed therebetween. The assembly is then fixed on the both sides thereof by a fastening bolt. This is an ordinary configuration of stuck fuel cell.
In this type of a solid polymer electrolyte fuel cell, the separator is required to meet the following requirements (1) to (5) In some detail, the separator needs to have a high electrical conductivity (requirement (1)). The separator needs to have a high tightness (requirement (2)). Further, the separator needs to have a high corrosion resistance to the reaction occurring during the reduction and oxidation of hydrogen/oxygen (requirement (3)). In addition, the separator also needs to have a heat resistance up to at least 100° C. (requirement (4)). This is because the fuel cell is normally operated at a temperature of around 100° C. or less. Finally, the separator also needs to have a high mechanical strength (requirement (5)). This is because MEA and the separator need to be fastened to each other at a face pressure of at least few kilograms-force per cm2 to reduce contact resistivity.
Since the separator needs to meet the aforementioned requirements (1) to (5), the related art separators have heretofore been conventionally formed by a carbon-based material such as glassy carbon and expanded graphite. Further, the gas flow path on the related art separator has heretofore been formed by cutting the surface of the separator or, if the separator is formed by expanded graphite, molding the gas flow path.
Recently, a separator prepared by compression-molding a mixture of graphite and a resin in a mold has been also used to reduce the cost of separator.
In recent years, an attempt has been made to prepare a separator by injection-molding a mixture of graphite and a resin (see JP-A-11-339823). This is because when the approach thus produced is conducted, the producing time can be reduced, making it possible to further reduce cost. When this approach is made possible, the producing facilities for the production of separator becomes simpler than the facilities for compression molding.
The entire disclosure of JP-A-11-339823 is incorporated herein by reference in its entirety.
Referring to the method of Preparation a separator by injection molding, as the separator material there is firstly prepared a compound comprising graphite and a thermoplastic resin in admixture. Subsequently, this compound is melt-kneaded in an injection molding machine. The compound thus melt-kneaded is then injected from the injection molding machine into the mold to form a separator. This method has been proposed.
Since the separator is required to have a high electronic conductivity, an approach is practiced to raise the proportion of electrically-conductive fillers in the compound. As the proportion of electrically-conductive fillers in the compound rises, the thermal conductivity of the compound rises and the fluidity of the molten compound during the injection from the injection molding machine falls. This results in extreme deterioration of moldability. This raises problems of underfilling, lack of strength at weld portions, etc.
In order to form a separator by injection molding rather than by cutting or compression molding, the following method and approach have heretofore been practiced. In some detail, referring to method, the injection molding material has heretofore been injected into the mold for separator from its periphery through a film gate or the like to mold separator. Referring to approach, the configuration of the flow path portion where the separator comes in contact with MEA has heretofore been almost the same as that of the separator prepared by cutting or like working.
Therefore, when the separator is prepared by injection molding, the molding material cannot sufficiently fill the mold for separator particularly at complicated flow path portions, giving some lack of uniformity in the material injected in the mold for separator. Further, since the separator material has a high thermal conductivity and thus cures quickly, weld portions can be easily produced in the mold for separator. Accordingly, the manifold portions disposed around the flow path on the separator has a reduced strength and a reduced air tightness to disadvantage. Thus, the preparation of separator by injection molding sacrifices the performance of the resulting fuel cell. In order to improve the fluidity of the molten material injected into the mold for separator, the configuration of flow path on the separator must be limited. Further, in order to compensate the low moldability during injection molding, the thickness of the separator must be raised. However, this improvement can cause the deterioration of cell performance or the rise of the dimension of the cell stack.
It is therefore an object of the invention to provide a mold for fuel cell separator which is assured a high dimensional stability, a high gas tightness, a reduced resistivity and a high mechanical strength, a fuel cell separator thus produced, a method and apparatus of producing same and a polymer electrolyte fuel cell comprising same.
It is another object of the invention to provide a mold for fuel cell separator capable of realizing enhanced reliability, enhanced cell performance and drastically reduced mass-production cost, a fuel cell separator thus produced by, a method and apparatus of producing same and a polymer electrolyte fuel cell comprising same.